Project 2 of this revised program project grant application involves functional neurochemical assessments using in vivo electrochemical recording techniques, microdialysis methods and high performance liquid chromatography coupled with electrochemical detection (HPLC-EC) to study dopamine (DA) neuronal systems in the striatum of young adult (5-9 year old), adult (13-16 year old) and late middle-age/aged (20-24 year old) Rhesus monkeys. This project will perform functional studies of neurochemical changes to the monkey striatum during aging in conjunction with the behavioral, histological and immunohistochemical studies of Project 1 and functional MRI-imaging studies in Project 3. First, in vivo electrochemical and microdialysis methods will be used to investigate potassium-evoked overflow of DA and p-tyramine-induced displacement of DA. These studies will be carried out in the caudate nucleus, putamen, and nucleus accumbens of the three age groups of monkeys. In addition to the evoked released of DA, basal levels of dopamine and dopamine metabolites will be studied using microdialysis. Secondly, both in vivo electrochemical and microdialysis studies will be used to investigate the functional properties of the dopamine transporter (DAT) located on dopaminergic nerve terminals. In vivo electrochemical methods will be used to look at DA uptake and microdialysis methods will be used to investigate the effects of GBR12909 and nomifensine on potassium-evoked release of dopamine in the striatum and nucleus accumbens of the young and aging monkeys. Project 1 will also provide histological measures of TH immunoreactive neurons. mRNA levels of the DAT, and cell counts of DA-containing neurons in the substantia nigra of the young adult and aging monkeys. In addition, Projects 1 and 3 will collaborate with this project to investigate motoric performance in the young adult and aging animals. In order to determine whether any age-related changes in motoric behavior and functional MRI imaging correlate with changes in the neurochemistry of DA, Finally, microdialysis measures of basal levels of DA and potassium-evoked overflow of DA will be performed in middle-aged monkeys that have received intraventricular (ICV) injections of vehicle or glial-derived neurotrophic factor (GDNF). In addition, studies of DA uptake will be performed using in vivo electrochemistry in the vehicle and GDNF-treated animals. These studies address the role of DA in motor behavior and the inherent plasticity and compensatory neurochemical properties of aging dopaminergic neurons in nonhuman primates.